A. Field of Invention
The present invention relates generally to fluid control valves and more particularly to valves used to control the pressure and/or flow direction of the fluid medium in hydraulic or pneumatic systems.
B. Description of Related Art
Direct acting pressure relief valves are available which employ a ball or a cross-sectionally circular poppet that is biased into sealing engagement with a valve seat surrounding the opening to a central flow passage, thereby preventing flow through the valve until and unless the pressure of the fluid medium exceeds a threshold level and then allowing flow in only one direction. When the force exerted by the pressure of the fluid medium on the ball or poppet exceeds the force exerted by the biasing means, the ball or poppet is disengaged from its seal with the valve seat which allows flow through the valve, thereby relieving and reducing the pressure within the system. The displacement of the ball or poppet from the valve seat results in the opening of a single orifice, typically annular, defined by the gap between the opposing surfaces of the ball or poppet and the valve seat. In the conventional direct acting pressure relief valve, the flow path of the open valve proceeds axially through the central flow passage, through the annular orifice between the ball or poppet and the valve seat, radially outward and around the ball or poppet and, in some configurations, thence radially inward into and then axially downstream through a central exit flow passage, typically the interior bore of the poppet. When the force exerted by the pressure of the fluid medium in the system is reduced to less than the force exerted by the biasing means, the ball or poppet returns to a sealing engagement with the valve seat and the valve closes. In the conventional pressure relief valves of this type compromises must be made in at least one of the following variables: the size of the assembled valve, the flow capacity through the valve, or the valve response characteristics, most commonly the valve hysteresis on opening or closing.
Commonly, in miniature axial flow valves for use in high pressure applications, acceptable rates of flow through the valve are achieved at the cost of relatively high closing hysteresis. The flow capacity of such valves is dependent upon the inside dimension of the central exit flow passage which must have a cross-sectional area in excess of that of the inlet flow passage at the valve seat to provide an adequate pressure differential across the valve. Since the outside dimension of the poppet must be greater than the inside dimension, such valves are characterized by a significant difference between the outside dimension of the poppet and the inside cross-sectional area of the inlet flow passage and the valve seat. In a valve of this configuration, the valve when open exposes to the inlet pressure of the fluid medium a greater area of the transverse aspect of the poppet surface than is exposed when the valve is closed, thereby increasing the speed at which the valve opens but conversely also tending to slow the closing of the valve by holding the valve open after the fluid pressure returns to a level below the cracking pressure. The increased transverse surface of the conventional poppet exposed to the inlet pressure on opening, may also cause instability under conditions where the flow rate is not adequate to sustain the valve in an open state.
Spring constraints are also common limitations in miniature valves that are required to maintain high flow capacities while subject to length and width restrictions, particularly when the valve must be inserted into the bore of the system. In such valves, the common poppet biasing means are springs and, in addition to the flow restraints imposed by the diameter of the valve, another primary size constraint is imposed by the size of the spring required for the desired application. Generally, shorter springs with higher spring rates can be used when the axial displacement of the poppet required to open the valve is shortened. In conventional direct acting relief valves, limitations shortening the stroke of the poppet are accommodated by increasing the diameter of the poppet to avoid decreasing the flow capacity of the valve when open. When the conventional valve is fully open, the stroke distance approximately equals the transverse area of the inlet flow passage divided by the circumference of the inlet flow passage. Consequently, in conventional direct acting valves, the stroke, and thereby the spring length, are ordinarily directly dependent upon the inside diameter of the inlet flow passage which in turn determines the flow capacity of the valve. The stroke and spring length determine the length of the valve body. Therefore, conventional single orifice valves are strictly limited by maximum ratios of flow capacity to valve size, and, particularly in aerospace applications where minimum size is essential, the challenge is to increase that ratio.